Think of the whitest bird you can imagine. Is it an egret? A swan? Maybe an owl?
A woodcock, its body mostly covered in browns and grays, might not seem like a contender. But it turns out that the whitest plumage known to science is found on the tip of a Eurasian Woodcock’s tail, according to a recent study.
Lead researcher Jamie Dunning says his jaw dropped when he first reviewed the results. “When we first got the data back, we’re all like, ‘This is wrong,’” says Dunning, an ornithologist at Imperial College London. “It’s just unbelievable to see this.”
Dunning’s team reached that surprising conclusion, published in March in the Journal of The Royal Society Interface, by comparing woodcock tail-tip feathers with plumage from 61 other birds, including bright-white species like Snowy Owl and Caspian Tern, both known for their white feathers. They did so by gauging the feathers’ diffuse reflectance, a measure of how an object scatters light in many directions, which determines how white the object will appear to human eyes. Woodcock feathers reflected up to 55 percent of light, the researchers found—about 30 percent more than has been measured in any other bird.
That astonishing whiteness is due to the combination of two factors: the microstructures of the tail’s white tip and how those structures are arranged. Using an electron microscope, the researchers found that a woodcock tail feather’s white ramus—the central axis of each feather barb—is abnormally thick while containing a sponge-like matrix of air and the protein keratin. This internal structure of the ramus allows all wavelengths of the light that enters it to be scattered evenly, making the feathers appear bright white to the human eye.
This particular structure has not been found in any other species, the researchers say. Other birds’ white feathers might have only some of the sponge-like materials, or a solid keratin layer with a hole in the middle, which still allows the light to be scattered. “But in woodcock, there’s just a very large amount of disordered material that’s not present in other birds, even other white birds,” says biologist and co-author Matthew Shawkey at Ghent University in Belgium.
The other factor is how the rami are arranged in a woodcock’s tail-tip feather: They are flattened and overlap each other like venetian blinds. The result is an increased surface area that reflects more light. And the angle at which the rami overlap—about 70 degrees—is the most efficient to further boost the reflectance of the feather, says Anvay Patil, a material scientist at the University of Akron who was also involved in the study.
Based on their findings, Dunning and his colleagues suggest that woodcocks, which are mostly nocturnal, might use their white patches to communicate with one another in the dark, as they do with sounds and chemical signals. The tail feathers are brown on the upper side, so the brilliant white doesn’t break their camouflage during the day. But when night falls, they can raise their tails and flash their white patches to interact with each other. “So they’re flashing this intense signal, then they’re putting it back down, and they’re invisible again,” Dunning says.
While Dunning’s study focuses mainly on the Eurasian species, American Woodcocks also have white patches under their tails that ornithologists believe may be comparably bright. Both birds have similar behavior: They mostly roost in the daytime, camouflaged in wooded or shrubby areas, and come out in the evening to feed on worms and to perform their mating display in relatively open fields.
A male woodcock’s mating display relies mostly on its peent calls and “sky dance.” But the birds sometimes raise their tails to flash the white feathers underneath, and researchers suspect that behavior could play a role in courtship. “I honestly never thought much about the visual signaling that they use,” says ornithologist Daniel Baldassarre at SUNY Oswego, “but it makes sense that it’s something that would give them a benefit evolutionarily.”
Another possibility for the brightness of the feathers, Dunning and his colleagues say, is that woodcocks might use their white undertail to distract predators. A male woodcock usually flies straight up and fans its tail when it senses danger closing in. When it opens its tail, there’s a chance the predators are drawn to the white patches rather than its body, according to Rachel Darling, an ecologist at the University of Maine who was not part of the study. However, she’s skeptical the tactic would work effectively for American Woodcocks, since their tail is smaller than the Eurasian ones. “If the predators are trying to grab the tail,” she says, “they might also get the body.”
Whether American Woodcocks have feathers with the same structure and brightness as their Eurasian cousins has yet to be confirmed, but we might learn the answer soon. Dunning’s study has intrigued Baldassarre, who says he’s now eager to look at some woodcock feathers under a microscope. “As soon as I get a chance to get into museum and get my hands on some American Woodcocks,” Baldassarre says, “I want to look and see how white they are.”